This invention relates to the casting of steel strip.
It is known to cast metal strip by continuous casting in a twin roll caster. In this technique molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term "nip" is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed.
Although twin roll casting has been applied with some success to non-ferrous metals which solidify rapidly on cooling, there have been problems in applying the technique to the casting of ferrous metals. One particular problem encountered in the casting of aluminium killed steel in a twin roll strip caster is the propensity for molten steel to produce solid inclusions, in particular inclusions which contain aluminates. Such inclusions can affect the surface quality of the strip as well as having the tendency to block any small casting passages in the metal delivery system. This has led to the use of manganese/silicon killed steels as an alternative, such as described in our New Zealand Patent Application 270147. However, such silicon/manganese killed steels have a significantly higher oxygen content than aluminium killed steels and this gives rise to problems in casters in which the delivery nozzle formed of refractory material containing carbon dips into the casting pool, the pool being disturbed by carbon monoxide bubbles generated by reactions between carbon in the submerged delivery nozzle and oxygen containing compounds in the molten metal of the casting pool. Such disturbance leads to the formation of discrete waves in the casting pool which are reflected in the cast strip as depressions in the strip surface. These defects are commonly referred to as meniscus marks. Moreover, carbon leaching from the refractory material of the metal delivery nozzle is enhanced.
Silicon/manganese killed steels will have an oxygen content in the range of 50-155 ppm at typical casting temperatures of the order of 1600-1700.degree. C. whereas the oxygen content of aluminium killed steels will generally be less than 10 ppm and the carbon leaching problem is a very significant one when endeavouring to cast silicon/manganese killed steel.
Our International Patent Application PCT/AU96/00244 describes a proposal to address this problem by the controlled addition of sulphur to the silicon/manganese killed steel melt at least in the start-up phase of a casting operation. However, the controlled addition of sulphur to the steel adds complexity to the process and results in the production of steel with high sulphur content which may not generally be acceptable to all markets. By the present invention the problem is addressed by modifying the chemical composition of the refractory material of the delivery nozzle rather than that of the steel melt.